1. Field of the Invention
This invention relates in general to orifice plate equipment for measuring gas flow in pipelines, and in particular to a seal for sealing the plate carrier of an orifice plate against the seat in an orifice fitting body.
2. Description of the Prior Art
One method of measuring gas flow in pipelines is by the use of an orifice fitting. The orifice fitting is an assembly that will be mounted for receiving gas flow. It has a body with a passage for the flow of the gas. A gear mechanism will lower an orifice plate down into the gas flow passages. The orifice plate has a hole through it. Gas flow through the hole causes a pressure drop. The difference in pressure upstream and downstream of the plate is used to calculate the gas flow velocity.
A plate carrier carries the orifice plate. The plate carrier has a seal which seals on the downstream side of the orifice plate. The seal seals against an annular seat which surrounds the flow passages. It is important to have good sealing so as to obtain an accurate measurement of the pressure drop.
In the prior art, the seal is an oversized elastomeric ring. It compresses as the plate carrier is forced into a slot between the seats. The slot will have a lesser width than the width of the plate carrier, including the seal.
A disadvantage of this technique is that the seal ring may be easily cut or damaged when forced into the seat slot. Also, variations in face-to-face dimensions between the seats as well as corrosion and erosion of the seat areas often cause leaks past the seal ring. In larger orifice fittings, unsupported segments of the seat area will sometimes expand beyond the yield strength of material during pressure testing. This causes a face-to-face dimension in this portion of the seat slot that is in excess of the allowable limits. This may result in leakage.
Because the leakage around the plate occurs only from an upstream to a downstream side of a plate, and not to the exterior of the fitting, it is difficult to detect. As a result, erroneous readings cause sellers of gas to lose large sums of money.